It is known in the pharmaceutical arts that low-solubility drugs often show poor bioavailability or irregular absorption, the degree of irregularity being affected by factors such as dose level, fed state of the patient, and form of the drug.
Solid dispersions of a drug in a matrix can be prepared by forming a homogeneous solution or melt of the drug and matrix material followed by solidifying the mixture by cooling or removal of solvent. Such dispersions have been known for more than two decades. Such solid dispersions of crystalline drugs often show enhanced bioavailability when administered orally relative to oral compositions comprising undispersed crystalline drug.
In general, it is known that the use of water-soluble polymers as the matrix material generally yields good results. Examples of water soluble polymers which have been employed include polyvinylpyrrolidone (PVP), hydroxypropyl methyl cellulose (HPMC), hydroxypropyl cellulose (HPC), methyl cellulose (MC), block copolymers of ethylene oxide and propylene oxide (PEO/PPO), and polyethyleneglycol (PEG). In a 1986 review of solid amorphous dispersions, see Ford, J. L., Pharm Acta. Helv., 61:3 (1986), criteria are set forth for choosing a suitable matrix, termed a “carrier” therein. The first and most important criterion listed therein is that the carrier “should be freely water soluble with intrinsic rapid dissolution properties.” As a result of this view, which is currently widely held, the majority of reports of solid amorphous dispersions of drugs in polymers use polymers which rapidly dissolve in water or gastric fluid such as PVP, PEG, or other water-soluble polymers.
There have been a relatively small number of reports of using water insoluble polymers as the matrix material for solid amorphous dispersions, although in some cases such polymers are soluble in aqueous base. The clear focus of most of these reports is on achieving sustained release of the drug, as opposed to increasing bioavailability. For example, sodium carboxymethylcellulose (NaCMC) and hydroxypropylmethyl cellulose acetate succinate (HPMCAS), both polymers that are insoluble in water or gastric fluid but soluble in aqueous base, such as solutions containing sufficient base to have a pH of 6.5 or greater following dissolution of HPMCAS, have been used in an attempt to simultaneously encapsulate and form a dispersion of drug via a spray-drying process. See Wan et al., Drug Development and Industrial Pharmacy, 18:9, 997-1011 (1992). The authors attempted to form a dispersion of theophylline in HPMCAS by dispersing crystals of theophylline and particles of HPMCAS in water. Neither the drug nor the HPMCAS dissolved appreciably in the water. The resulting slurry was spray dried and resulted in a product (p.1009, line 11) consisting of long thin needle-like theophylline with scattered HPMCAS particles. The authors concluded (p.1010, line 5) that of the polymers studied, only HPMCAS was found unsuitable for their process. The authors state that the intent of the process was to retard rather than enhance the rate of release of drug. Indeed, for all polymers disclosed, in vitro tests showed drug concentrations that were the same or lower than that obtained with drug alone.
Miyajima et al., U.S. Pat. No. 4,983,593, disclose, inter alia, formulating HPMCAS with a drug designated as NZ-105. The patent disclosed that there is formed “a composition having a remarkably enhanced bioavailability and easily prepared into tablets, capsules, granules, powders, and the like . . . .” The patent teaches that the formulations can be prepared by dissolving NZ-105 and HPMCAS in an organic solvent and removing the solvent by means of vacuum-drying, spray-drying, freeze-drying, or the like, or by coating a filler such as an inorganic salt (e.g., calcium hydrogen phosphate) or a sugar (e.g., lactose, sucrose, and so forth) and the like by means of a fluidized bed granulation method, a centrifugal coating method, or a pan coating method to produce granules. The patent discloses that granules can also be prepared by adding a solvent to a filler and kneading the mixture followed by drying. All examples in the patent describe forming a dispersion of HPMCAS and NZ-105 by either (1) fluidized bed granulation by coating either calcium hydrogen phosphate particles or lactose crystals to form large particles up to 1400 μm in diameter or 2) vacuum drying with lactose to form a solid cake that is then pulverized to form a powdery material.
Nakamichi et al., U.S. Pat. No. 5,456,923, disclose, inter alia, a process for producing solid dispersions by passing a mixture of a drug and a polymer carrier through a twin screw compounding extruder. HPMCAS is mentioned as one polymer from among a group of suitable polymers which can be used.
U.S. Pat. No. 5,456,923 to Shogo et al discloses an extrusion method for making solid dispersions. HPMCAS is included in a list of polymeric materials, including materials such as starch or gelatin, that can be used as matrix materials.
Takeichi et al, Chem. Pharm. Bull, 38 (9), 2547-2551 (1990) attempted to use a solid dispersion of HPMCAS and uracil made by grinding in a ball mill to enhance rectal absorption, but concluded that uracil absorption was lower than for low molecular weight matrix materials such as sodium caprate. The use of HPMCAS was not recommended.
Baba, et al, Chem. Pharm. Bull, 38 (9), 2542-2546 (1990) made ground mixtures of uracil and HPMCAS along with 50 other matrix materials. Although some enhancement (about a factor of 2) in the dissolution of uracil was observed in the co-ground HPMCAS material relative to a simple mixture of crystalline drug and HPMCAS, the enhancement decreased as the polymer-to-drug ratio was increased. This led the researchers to conclude that HPMCAS adsorbed on the surface of the uracil thereby hindering the dissolution of uracil. Its use was not recommended.
T. Yamaguchi et al, Yakuzaigaku, 53 (4), 221-228 (1993) prepared solid amorphous dispersions of 4″-0-(4-methoxyphenyl)acetyltylosin (MAT) in HPMCAS as well as CMEC. Dissolution tests at pH 4.0 showed supersaturated concentrations of MAT 9-fold that of crystalline MAT with HPMCAS dispersions. This concentration was comparable to that obtained with the dissolution of amorphous drug alone. However, the presence of HPMCAS sustained the supersaturation longer than the amorphous drug alone. The authors report that even better results were obtained with the CMEC dispersions, however, causing the authors to conclude that CMEC is the preferred dispersion matrix.